A question I answer a fair amount in my job. What are they usually asking about? Anemones. "Well, they're animals. They're relatives of jellyfish and corals." "OK...huh? So...how are they animals?"
This next question, the how or why are they animals question, is more difficult to answer. Anemones, fortunately eat and move so you can tell people that without having to get into the difference between a cell wall and cell membrane. Something else I find myself trying to explain a fair amount is the fact that seaweeds are not plants.
Seaweeds are protists in the family that eventually led to land plants so calling them plants is a bit like calling an amoeba an animal. The kingdom protist eventually led to all land plants, animals and fungus so it's a very diverse group that has been around for millions of years. Most protists are unicellular but things like kelp are multicellular protists. Seaweeds typically get energy from sunlight the way plants do but they have some serious structural differences. They don't have roots, leaves, stems, flowers or any of the parts we associate with plants.
The matter is further confused when you introduce the term algae. Algae is generally used to describe seaweeds (plant-like protists) that are unicellular and generally collect on surfaces (surfaces of rocks or surfaces of the water or that junk on your home aquarium). Now most algaes are protists but some are...actually bacteria. Bacteria are not even in the same family at all. They're just something, while alive, very different from eukaryotes like plants and animals and seaweed.
So why do I care about this? Well I think it's inherently interesting. But I think the reason I want to teach people about this confusing nonsense is that I believe it makes one appreciate how much more complex the living world is than you might first imagine. Whenever we talk about "animals" vertebrates get the lion's share (get it?!) of the air time. Vertebrates are one small family accounting for something like 4-5% of living animal species. Similarly, whenever we talk about animals and plants we're missing protists, which themselves are incredibly diverse, fungi, cyanobacteria, and y'know, archaea which is a whole kingdom. Archaea may not be terribly diverse, but still we're missing a whole kingdom most folks don't even know exists.